Centrifugal compressors are useful in a variety of devices that require a fluid to be compressed, such as chillers. The compressors operate by passing the fluid over a rotating impeller. The impeller works on the fluid to increase the pressure of the fluid. Because the operation of the impeller creates an adverse pressure gradient in the flow, some compressor designs include a variable geometry diffuser positioned at the impeller exit to stabilize the fluid flow during stall events, thereby mitigating stall. Stall results as refrigerant flow decreases while the pressure differential across the impeller is maintained. Stall undesirably creates noise, causes vibration and reduces compressor efficiency.
Since stall conditions are present only a very small percentage of the time that the compressor operates, the operation of the variable geometry diffuser similarly has been limited, so that wear and tear, loadings and other functions that affect the overall life integrity of a diffuser mechanism has been limited. However, increasing usage of a variable geometry diffuser mechanism would dramatically affect the overall reliability and life of a diffuser mechanism.
A diffuser design that has been effective is set forth in U.S. Pat. No. 6,872,050 issued on Mar. 29, 2005, to Nenstiel (the '050 Patent). The '050 Patent discloses a variable geometry diffuser that is opened and closed during the operation of the compressor, is inexpensive to manufacture, is easy to assemble, is simple to repair or replace, and provides positive engagement for position determination in response to signals or commands from the controller in response to incipient stall conditions.
The variable geometry diffuser design of the '050 Patent utilizes a diffuser ring movable between a first retracted position in which flow through a diffuser gap is unobstructed and a second extended position in which the diffuser ring extends into the diffuser gap to alter the fluid flow through the diffuser gap in response to detection of stall. This is accomplished by extending the diffuser ring substantially across the diffuser gap to alter fluid flow. This mitigation can be accomplished by extending the diffuser ring across about 75% of the diffuser gap. The diffuser ring is driven by a drive ring movable from a first position corresponding to the first retracted position of the diffuser ring, a second position corresponding to the second extended position of the diffuser ring, and any intermediate position between the first position and the second position. The second position is an extended position that stabilizes the system at about 75% of the diffuser gap so that stall is mitigated. The drive ring in turn is mounted to support blocks, and the drive ring is rotationally movable with respect to the support blocks, which are mounted to the backside of a nozzle base plate. The nozzle base plate is fixed to the housing adjacent the impeller of the centrifugal compressor. While the variable geometry diffuser design is effective during compressor operation in altering flow through the diffuser gap when the diffuser ring is in its second extended position, the diffuser ring does not sufficiently block flow during compressor shutdown to retard compressor backspin and associated transient loads or to avoid transient surge and stall during start-up as the compressor ramps up from low loads and low speeds to high speed.
Use of the variable geometry diffuser generates a load on the diffuser ring due to a pressure differential on the overall ring area. When the ring is in its retracted position, the compressed refrigerant passes over the ring surface and very little load is encountered. However, as the ring moves to its extended position into the diffuser gap, high velocity gas passes over the face of the diffuser ring creating a low pressure area. Higher pressure gas in the groove of the nozzle base plate exerts a force on the back side of the ring. The load on the ring, and the rest of the variable geometry diffuser mechanism, can be calculated. It is the difference in gas pressure on either side of the ring multiplied by the area of the ring. The variable geometry diffuser of the present invention includes a relatively large diffuser ring, the operation of which must overcome substantial forces and which must withstand substantial forces in operation. Thus, the mechanisms are substantial and the energy required to operate these mechanisms to overcome these forces is also substantial. However, because the variable geometry diffuser is engaged for only a small percentage of the overall life of the compressor, the loads and wear and tear experienced by the variable geometry diffuser have been acceptable.
There is a desire to increase the usage of the variable geometry diffuser ring so that it can be used for more than as just a stall mitigation device. The variable geometry diffuser may be used for not only stall mitigation, but also for capacity control, surge control, improved turndown, minimization of compressor backspin and associated transient loads during compressor shut down as well as for minimization of start-up transients. Due to the increased usage of such a variable geometry diffuser, an improved device is required to provide desirable control enhancements to overall centrifugal compressor operation, while providing longevity to the variable geometry diffuser experiencing increased usage.